FIG. 1 is a generalized diagram of an optical bistable element using a conventional p-i-n photodiode described by David A. B. Miller et al., in IEEE Journal of Quantum Electronics, QE-21, No. 9, September 1985, Pages 1462-1476. A p-i-n photodiode 20 comprises an AlGaAs/GaAs multiple quantum well layer 1, undoped AlGaAs layers 31 and 32 respectively disposed on the opposed surfaces of the multiple quantum well layer 1, a p-type AlGaAs layer 4 disposed the undoped AlGaAs layer 31, an n-type AlGaAs layer 5 disposed on the undoped AlGaAs layer 32, and electrodes 82 and 81 respectively disposed on the p-type and n-type AlGaAs layers 4 and 5. An external voltage source 11 providing a voltage Vex is coupled between the electrodes 81 and 82 through an external resistor 9 having a resistance value of R. The external voltage source 11 is connected to reverse bias the p-i-n diode 20.
The operation of the optical bistable device using the p-i-n diode 20 shown in FIG. 1 is explained with reference to FIGS. 2 and 3.
The reverse bias voltage Vex is applied through the external resistor 9 to the p-i-n photodiode 20. The multiple quantum well layer 1 within the photodiode exhibits a sharp peak in the absorption spectrum due to exciton absorption corresponding to quantum level transitions. The peak in the absorption spectrum can be shifted by changing the internal electric field. In other words, the peak absorption coefficient at a given incident light wavelength depends on the internal electric field.
Light incident on the photodiode 20 is absorbed by the multiple quantum well layer 1 and corresponding photocurrent I is generated. FIG. 2 shows relationship of the light absorption coefficient of the photodiode 20 to a voltage applied to the photodiode 20. It is seen that the light absorption coefficient is at a peak when the voltage applied to the photodiode 20 is at a particular value. In the arrangement shown in FIG. 1, a voltage drop occurs across the external resistor 9 due to the photocurrent I, which causes the voltage V applied to the photodiode to change from Vex to Vex--IR. The relationship between the photocurrent I and the absorption coefficient S can be expressed as: EQU I=.alpha.SP.sub.in ( 1)
where .alpha. is a constant, and P.sub.in is an incident light intensity. From this, the absorption coefficient is expressed as: EQU S=(Vex-V).alpha.RP.sub.in ( 2)
Thus, the absorption coefficient S may be represented by a line with a slope which decreases as the intensity of incident light P.sub.in increases. In FIG. 2, lines A-D are lines which can be expressed by the equation (2) for different intensities P.sub.in of incident light. An actual operating point is at an intersection of this line with the curve showing the relationship between the voltage V applied to the photodiode 20 and the light absorption coefficient S of the photodiode 20.
As will be understood from FIG. 2, the lines A and D intersect the curve representing the absorption coefficient S at only one point, but lines lying between the lines B and C intersect the curve at three points. The central one of the three points is an unstable point, and the remaining two are stable points. That is, the element having those lines which lie between the lines B and C are bistable.
In FIG. 3, relationships of the output light intensity P.sub.out to the incident light intensity P.sub.in in is shown. Bistability is exhibited within a range of the incident light intensity P.sub.in. Specifically, as the intensity of incident light increases, the output light intensity abruptly decreases at a certain value of the incident light intensity, and when the incident light intensity is decreased from a value larger than the value at which the abrupt decrease of the output light intensity occurs, the output light intensity will increase abruptly at a certain value of the incident light intensity, as shown in FIG. 3.
However, conventional techniques cannot provide optical bistable elements in which the output light intensity P.sub.out abruptly increases at a first value of the incident light intensity P.sub.in as the incident light intensity is increased, and abruptly decreases at a second value of the incident light intensity P.sub.in as the incident light intensity is decreased.
Since conventional optical bistable elements have such characteristics as stated above, it is difficult to provide an optical multi-stable characteristic having three or more stable points. Furthermore, more than one optical stability characteristic cannot be obtained.
An object of the present invention is to provide an optical multi-stable device which can exhibit a plurality of optical multi-stable characteristics for an incident light wavelength or an incident light intensity.